The present invention relates to an apparatus and method for scanning (also referred to as reading) information that is stored in a memory-retaining luminescent material (hereinafter referred to as a memory phosphor).
In particular in medical applications, an image of an object, for example a patient, is produced by means of x-rays and stored as a latent image in a memory phosphor. The latent x-ray image is retrieved by stimulating the memory phosphor by means of a radiation source. In response to the stimulant radiation, the memory phosphor will emit light of an intensity that is proportional to the amount of x-ray radiation stored in the memory phosphor. The light emitted by the memory phosphor (hereinafter referred to as emitted radiation or secondary radiation) is received by a detecting device and converted into electrical signals, so that the x-ray image stored in the memory phosphor can subsequently be made visible. The x-ray image can be displayed, for example, directly on a monitor, or it can be transferred photographically to a radiographic film.
The memory phosphor provides a large dynamic range for the intermediate storage of x-ray information. However, the extent to which this dynamic range can be used may be limited depending on the kind of detecting device that is employed for the retrieval of the x-ray information from the memory phosphor. This limitation becomes apparent in particular when examining objects that differ widely in the dose of radiation required for the x-ray image.
A device for reading information stored in a memory phosphor is known for example from the patent application WO 99/28765. The device uses an arrangement where the memory phosphor is excited line-by-line by a stimulant radiation that is produced by a radiation source. The radiation source may be configured, e.g., as a line of laser diodes. The light emitted by the memory phosphor as a result of the stimulation is received by a detecting device. To perform its receiver function, the detecting device has a large number of radiation-sensitive surface segments that are arranged in a row next to each other. The radiation emitted by the memory phosphor is received by the radiation-sensitive surfaces. The detecting device can be a linear CCD (Charge-Coupled Device) array containing a large number of photo-detectors arranged in a row next to each other. To take the reading from the memory phosphor, the stimulant radiation source and the detecting device are moved at a uniform speed across the memory phosphor. In response to the amount of emitted radiation that each of the radiation-sensitive surface segments receives during an exposure period, the detecting device produces a corresponding number of electrons. Thus, the electrostatic charges produced for each of the pixels of a row represent a measure for the amount of radiation emitted by each individual pixel.